How to validate constraints of ManyToOne relations?

Question

I've got two models with 1:n relation and want to use form validators.

If I choose an option from the select, the correct id of the element is stored on the database.
But if I leave it -- Choose a game--, NULL is stored although the game property is annotated with @Constraints.Required.

Models

@Entity
public class Server extends Model {

    @Id
    public Integer serverId;

    @ManyToOne
    @Constraints.Required
    public Game game;

    ...
}

@Entity
public class Game extends Model {

    @Id
    @Constraints.Required
    public Integer gameId;

    @Constraints.Required
    public String name;

    public static Map<String,String> options() {
        LinkedHashMap<String,String> options = new LinkedHashMap<String,String>();

        for(Game c: Game.find.orderBy("name").findList()) {
            options.put(c.gameId.toString(), c.name);
        }

        return options;
    }

    ...
}

Controller

public static Result create() {
    return ok(views.html.Server.create.render(form(models.Server.class)))
}

Template

@helper.select(form("game.gameId"), helper.options(Game.options), '_default -> "-- Choose a game --")

---

I've already tried to use a @Valid annotation to force play.data.Form to validate the Game-Model during the binding process, but that only causes a simple evaluation of the constraints on Game properties, which are not filled. Although a correct game is choosen from the list, it gives me an error that name is empty:

@Entity
public class Server extends Model {

    @ManyToOne
    @Constraints.Required
    @Valid
    public Game game;

    @Constraints.Required
    public String name;

...

---

Thanks for your help.

Answer 1

I fixed this problem by writing a validate() method like this:

public boolean validate() {
    return Game.gameId != null
}

I also looked into the Spring Binding documentation, because Play2.0 Java uses the Spring Binder. In chapter 17.2.4.11 (link) they're giving the default option the value "-". Haven't tested it, but it might be the solution.